Fabrication of practical zeolite membranes has long been a goal of separation science. For a zeolite membrane to be practical, it must have a high flux and selectivity for the desired permeate molecule(s). Obtaining such a membrane has been difficult because of defects and interparticle voids inherent to the zeolite film. This is especially true for membranes grown from classical zeolite synthesis routes described in the literature. These membranes have a heterogeneous crystal structure within the membrane layer and require an enormous (&gt;50 microns) layer thickness to seal pinholes and void structures. What is needed in the art is a thin, continuous zeolite layer with few defects, and a method, or methods, to `heal` any remaining defects and void structures and stabilize these to subsequent severe conditions of operation.
Membranes described in the literature are formed with several zones (larger crystals grown on top of smaller crystals) across the membrane thickness. In several zones, the crystals are not grown into a dense mat that is free of intercrystalline voids To obtain a permselective zeolite membrane, the above zeolite layers (comprised of zones) must be grown to an excessive thickness (&gt;50 microns) to seal off voids and defects within the membrane system. This introduces a great mass transfer resistance resulting in very low fluxes through the membrane layer.
Alternately, a method (or methods) to `heal` void structures and defects in the membrane layer, without having a negative effect on permeate flux or selectivity, would eliminate the need to grow excessively thick zeolite layers. Most successful membrane technologies employ some form of reparation coating which repairs existing defects or holes in membranes. Reparation techniques either selectively seal defects and holes or attenuate their effects with a permeable coating that covers the membrane surface. Reparation techniques which selectively seal defects are often based on the selective application of a film forming material which acts as a diffusion barrier. Reparation technologies which attenuate rather than selectively seal defects or holes apply a permeable layer over the entire surface of the membrane. The mass transfer resistance of this layer must be sufficient to attenuate the effect of defects in the membrane and improve the permselective properties of the membrane. In all events reparation technologies improve the permselective properties of membranes with existing defects or holes. Defects and holes form nonselective permeation pathways through the membrane and reparation decreases the flux through these nonselective pathways relative to permselective pathways through the reset of the membrane.
A large number of reparation coating technologies have been developed for organic membranes. Because of differences in materials, conditions of use and physics of the transport mechanism through different membranes, reparation coatings developed for organic membranes cannot be used with any predictability regarding reporting inorganic and zeolite membranes. Coating materials which have been used to reparate polymer membranes have been generally polymer and epoxy materials. When they are used to reparate polymer membranes by selectively sealing defects, film forming polymers or epoxies are applied in a manner such that they are selectively absorbed into defects or holes. For example, hollow fiber membrane modules containing a small percentage of broken fibers have been reparated by selectively filling broken fibers with epoxy. When they are used to attenuate defects, permeable polymers or epoxies are applied as a thin film coating over the organic membrane surface. The polymer and epoxy materials used to seal and attenuate defects in polymer membranes degrade in the harsh chemical environments and high temperature operations in which inorganic and zeolite membranes are used.
Methods used to reparate organic membranes are not likely to be applicable to microporous or mesoporous inorganic membranes. This is especially true for zeolite membranes for two reasons. First, since materials used to reparate organic membranes are organic in nature, wettability is an issue with oxide materials such as zeolites, or any system that is heavily hydroxylated at the surface. It is not obvious that an organic polymer would adhere to the zeolite or support surface much less remain intact to seal voids/defects. Secondly, zeolite membranes take advantage of the well defined pore structure of zeolitic materials, reparation using organic polymers may seal these pores in addition to the desired voids/defects; it is not likely that the material will be able to discriminate between void structures or defects and zeolite pores. If the coating material enters the pore structure of the zeolite, it can occlude or block the pore structure. When it occludes the pore structure, it hinders diffusion through the zeolite and no transport occurs through blocked pores. Even when the coating material does not enter the pore structure, it can occlude pore mouths at the surface of the zeolite and hinder diffusional transport through the membrane. There have been proposals that zeolite membranes grown by hydrothermal synthesis could be reparated by deposition of a material that is different from that used to form the zeolite layer in the membrane (see for example EP 481,660A1, S.A.I. Barri and G. J. Bratton, and T. de V. Naylor, British Petroleum, 1991), however no specific recipe for reparation has been presented.